Using a regular head pillow between the knees is reported to reduce back pain by the Mayo Clinic and many other sources. The use of head pillows between the knees and the use of specially adapted knee pillows are well known, and many different models of adapted knee pillows are commercially available. The general purpose is to provide leg separation at the knee for people sleeping on their sides so as to provide for greater comfort, less joint strain and better circulation than when the knees are allowed to come together.
A loose pillow placed at the knee will fall out of place when a person changes sleeping position from one side to the other. Most adapted knee pillows are contoured to help keep the pillow in position, and some knee pillows are provided with straps for attaching the pillow to a leg. In this way, the pillow may stay on even when the user rolls over in the night.
An example of a contoured pillow that attaches to the leg is U.S. Pat. No. 4,177,806 to Griffin that attaches to one leg both above and below the knee. The pillow is curved to follow a typical sleeping position with the knees slightly bent. Another example of a knee pillow that attaches to the leg, but is not contoured, is described in U.S. Pat. No. 6,145,508 to Seip that attaches both above and below the knee, and provides more rigid foam or padding above and below the knee, while having a flexible batting material at the knee itself so that a user can bend the knee as she wishes during sleep.